1. Field of the Invention
The present invention relates to a still picture recording apparatus for recording a desired picture from video signals as a still picture.
2. Description of the Background Art
The recent advancement in digital technology is remarkable also in the video field, and it is now possible to record and process a video signal, which is an analog quantity, without lowering its quality by digitizing it.
One of such examples is the so-called digital still video tape recorder, which is a still picture recording apparatus for recording the video data of one screen of a television receiver as a still picture, in a magnetic tape which is a recording medium differing in transmission rate.
In the case that the recording frequency of the recording medium used in the still picture recording apparatus is lower than the carrier frequency of the video signal, the video signal is analog-to-digital converted (A/D converted), and the signal is once stored in means for storing, such as a memory, and then it is transmitted and recorded in this apparatus at a lower transmission rate.
Let us then suppose to record NTSC video signals (hereinafter referred to merely as video signals) in a magnetic tape by using a digital audio tape recorder (DAT apparatus). The time per screen of a video signal is 1/60 second, and assuming the number of quantitized bits for A/D converting the video signal to be 8, and the sampling frequency to be four times the video subcarrier frequency (fsc=3.58 MHz), that is, 4 fsc=14.32 MHz, the number of data bits per screen is calculated as follows. EQU 1/60.times.8.times.14.32.times.10.sup.6 =1.911 megabits (1)
On the other hand, the DAT apparatus deals with ordinary audio signals, and supposing the number of quantitized bits to be 16, the sampling frequency to be 48 kHz, and the number of channels to be 2, the number of bits that can be recorded in one second is obtained as follows. EQU 16.times.48.times.10.sup.3 .times.2=1.536 megabits/second (2)
Therefore, the time required for recording one still picture on a magnetic tape by using the DAT apparatus is EQU 1.911/1.536=1.244 seconds (3)
Hence, to record one still picture by DAT, the video data cannot be taken in unless an interval of 1.244 seconds or more is provided.
FIG. 1 is a block diagram of a conventional still picture recording apparatus 21. The still picture recording apparatus 21 is intended to record the video information for one screen from video signal V onto a magnetic recording paper (hereinafter called tape) 26, and it comprises, among others, an A/D converter 22 for converting video signal V into a digital signal, a still picture memory 23 for storing digital data for the portion of one screen, and a recording signal processing circuit 24 for reading out the stored data at the transmission rate of DAT, processing it by parity addition, modulation or the like, and recording it onto the tape 26 through a recording head 25.
A gate pulse generator 27 individually generates, when an operation switch 28 is pressed, a first gate pulse signal d21 and a second gate pulse signal d22 to a video system clock signal generator 30 and a tape system clock signal generator 31 at the timing of the vertical synchronizing signal Sv obtained from the video signal V through a synchronous separating circuit 29.
The video system clock signal generator 30 generates a control clock signal cka used at the time of writing into the A/D converter 22 and still picture memory 23. The tape system clock signal generator 31 generates a control clock signal ckb at the time of writing into the still picture memory 23 and recording signal processing circuit 24.
FIG. 2 is a time chart showing the operation of the conventional still picture recording apparatus 21 shown in FIG. 1. In FIG. 2, the parts corresponding to those in FIG. 1 are identified with the same reference codes.
When the operation switch 28 is pressed, the potential of a trigger terminal g of the gate pulse generator 27 becomes zero at time T0 as shown in FIG. 2 (1), and at the immediately succeeding timing of vertical synchronizing signal Sv, a first gate pulse signal d21 with time width t21 corresponding to the portion of one screen from time T1 is delivered as shown in FIG. 2 (2). This time width t21 is 1/60 second, and for this period, the video system clock signal generator 27 operates, and the video data for one screen converted digitally is taken into the still picture memory 23.
At time T2 when the level of the first gate pulse signal d21 becomes zero, a second gate pulse signal d22 shown in FIG. 2 (3) is delivered for the period of 1.244 seconds as shown in formula (3), and the tape system clock signal generator 28 is actuated, and the data of one screen portion stored in the still picture memory 23 is read out and recorded in the tape 26. That is, at the time of pressing of an operation switch 28, a still picture is selected from the video signal V, and is recorded in the tape 26.
However, in the still picture recording apparatus of the prior art, since pressing of the operation switch 28 is needed for recording, a person (an operator) must always monitor the screen, and press the operation switch 28 every time the desired picture appears. To be liberated from such obligation, it has been proposed to record the still picture automatically by applying a pulse signal of a specific period instead of as a result of operation of switch 30. However, this results in the problem of failure to record a picture truly desired to be recorded while recording the still picture.